Preparation of lithographic printing plates



United States Patent PREPARATION OF LITHOGRAPHIC PRINTING PLATESApplication October 26, 1953, Serial No. 388,438

3 Claims. (Cl. 955.4)

N Drawing.

f This invention relates to the preparation of stabilized syntheticpolymer sensitized inc plates useful for preparing lithographic printingplates and more particularly to such plates sensitized with cinnamicacid esters of polyvinyl alcohol.

1n the Minsk et al. U. S. Patent 2,610,120, granted September 9, 1952,and the Minsk et al. U. S. patent applications Serial Nos. 207,0852,filed January 20, but, Serial Nos. 207,049; 207,050 and 207,051, nowPatent Nos. 2,610,285; 2,670,286 and 2,670,287, respectiveiy, aredescribed methods for the preparation of light-sensitive cinnamic acidesters of polyvinyl alcohol and the sensitizing of compositionscontaining such esters with various compounds such as nitro-varyl,quinone, anthrone, and triphenylmethane compounds and oiamino mpnenylcarbmol, oiamino diphenyl methane and diammo diphenyl ketone compounds.Such light-sensitive esters are disclosed as useful in the preparationof resist images on metal plates such as zinc plates for printingpurposes. in such usage, the lightsensiiive compositions are coated fromsolvent onto a metal surface and are exposed under the desired subjectssuch as a line or halftone subject followed by removal from the plate ofthe unexposed regions with solvent to yield a light-hardened resistimage on the metal surface.

it has been discovered that in general, solvent solutions of thelight-sensitive polymeric cinnamic acid esters are stable and in theabsence of light do not undergo insolubilization, and generally when thecompositions are coated on various surfaces do not become insoluble inthe absence of light. However, when the compositions are coated on zincplates, initially, within a period of about 25 hours, satisfactoryresist images can be formed on the zinc plates by the above method butafter about 24 hours it is found that even in the dark, the zinc platesapparently have sufliciently changed the solubility of the resinimmediately adjacent to the surface of the zinc plate that the qualityof the printing plate is decreased. "that is, the printing ink adheresto the unexposed areas of the plate ostensibly free of polymeric resist.This effect is particularly noticeable when the cinnamic acid esters ofpolyvinyl alcohol which were used in coating the zinc plate have beenespecially sensitized with the above-mentioned compounds. "lhe eifectis'manifested by the formationof a very thin stratum of sparinglysoluble reaction product (resin or other substance) on the surface ofthe plate, which product is not readily removed by the solvent developerin the unexposed areas which should repel the printing ink, but whichtend to attract the ink. This condition is called scumming. 1n the casewhere etched zinc plates are being prepared by forming the resist on thezinc surface followed by etching out those areas not occupied by theresist, the effect of the presence of the insoluble reaction product onthe surface of the zinc is to impede the etching process. Consequently,the quality of prints obtained from such zinc printing platesprogressively decreases with increase in age of the polymer coating onthe Zinc plate. The mechanism of the reaction at the surface of the zincis not evident.

The Smith et al. U. S. patent application Serial No. 266,236, filedJanuary 12, 1952, describes incorporating an insolubilization inhibitorsuch as hydroquinone into the polymeric cinnamic acid ester coatings toprevent scumming. That procedure is partially effective in pre- 2venting scumming but I have discovered that a further improvement can beeffected by incorporating about 0.05 to 0.15 gram of triethanolamine in100 cc. of a 3.75 percent solvent solution of the cinnamic acid estersbefore coating on the zinc surface. I have found another procedure whichmakes the processed image more hydrophobic and thus increases theink-water differentiation between the image and the background areas.This enables the use of more fountain solution for a given ink-carryingcapacity and so acts to decrease scumming. The process involves treatingthe zinc plate upon which the insoluble cinnamic acid ester resist hasbeen formed, with a solution of iodine in carbon tetrachloride. Asimilar treatment of polyvinyl cinnamate resist images with an aqueoussolution of alkali metal permanganate,

T however, gives a surprising reversal of inking properties.

That is, the polyvinyl cinnamate resist becomes waterreceptive. Thisresult allows the preparation of a cinnamic acid ester resist image onink-receptive surfaces such as copper in the usual manner and treatmentof the plate with the permanganate solution produces a positive-positiveplate in which the metal areas carry the ink and the cinnamic acid esterresist areas repel the printing ink in the presence of moisture.

I have now discovered a procedure for removing this scum on zincsurfaces provided with cinnamic acid ester resist images which is veryeffective. The process is very simple but surprising in its results.After the formation of the insoluble cinnamic acid ester resist image ona zinc surface, I merely swab the plate with either ethylene glycolmonomethyl ether or ethylene glycol monoethyl ether. The chemical orphysical mechanisms involved are not clearly understood, however, itappears that these compounds are capable of removing from the surface ofthe zinc ,a thin film of reacted polymer which is responsible for thescumming tendencies of the plate. These two compounds are not solventsfor the original lightsensitive cinnamic acid ester itself and scummingis not prevented if these two compounds are used initially for thedevelopment of the resist image on the zinc surface. Moreover, solventmixtures of these compounds with other solvents for the polymer are noteifective in preventing scum when used for initially developing theresist image. It appears essential that the exposed lightsensitive layerbe first developed with a solvent in which the unexposed polymer ishighly soluble, such as hydrocarbon solvents, for example, toluene andxylene, aliphatic ketones such as methyl ethyl ketone, aromatic ketones,for example, acetophenone and especially halogenated hydrocarbonsolvents such as ethylene dichloride, chlorobenzene andtrichloroethylene. After thus developing the polymeric resist image onthe zinc surface, I then swab the plate with either ethylene glycolmonomethyl ether or ethylene glycol monoethyl ether. A similar compound,ethyleneglycolmonomethylether acetate, is a good solvent for developingthe initial resist image. However, this compound is not a solvent forthe reacted polymer causing scum and, therefore, does not remove scumfrom the printing plate.

Example A cinnamic acid ester of polyvinyl alcohol containingapproximately mol percent of recurring vinyl cinnamate units (obtainedby the esterification of polyvinyl alcohol with cinnamoyl chloride bythe process of Minsk et al. patent application Serial No. 207,050, filedJanuary 20, 1951), was provided. A 2.5 percent dope of the polymer inethylene glycol monomethyl ether acetate was prepared and a suitablesensitizer added such as 1,2-benzanthraquinone,4,4-tetramethyldiaminodiphenyl ketone or1-methyl-2-benzoy1methylene-,L-3-naphthothiazoline. The dope was thencoated upon a zinc-coated aluminum plate. A plain or grained zinc plateis also satisfactory. The coated plate was then exposed under a line orhalftone subject (so-called two-tone subjects) until the polymer coatingwas insoluble in the exposed area. The resist image was then developedwith an active solvent such as those mentioned, preferably by treatmentwith trichloroethylene vapor. The freshly coated plate carrying thepolymeric resist image as thus developed showed a slight tendency towardscumming after treatment with'a'conventional gum-free desensitizing etcand printingon a lithographic press. However, when the trichloroethylenedeveloped plate was swabbed with ethylene glycol monoethyl ether beforeapplying the desensitizing etch, there was no evidence of scurnming, Theresults of the treatment with ethylene glycol monoethyl ether were muchmore pronounced when applied to a plate carrying a coating of the samepolymer which had been aged 'on the platetojrv several months beforeexposure and development of the resist image This aged 'plate showedvery bad scurnming when developed with trichloroethylene, treated 'withthe desensitizing etch, and printed; whereasanother sample of the sameaged coating on the zinc-coated aluminum which had been exposed,developed "with,tr'ichloroethylene, s wabbed with ethylene glycolmonoethyl ether, fdesensitizedf. and printed, showed 'no evidence ofscum'rning. A similar relslult was obtained using ethylene-glycolinoriometh'yl et er.

T glycol er t ati g s ep f y n ent on i applicable to the removal ofscum 'nzj c surfacescoated with various cinnamic acid esters [ofpolyvinyl alcohol; f r. x p P y inyl nnama Po y i yl. hl rocinnarnate'and polyvinyl m nitrocinnamate The useful ester should in generalcontain at least 60. percent Gin namoyl ester groups, the remaininggroups in "the polymer may, for example, by combined vinyl acetate orvinyl alcohol units. The cinnamic acid 'e'ster of polyvinyl alcohol towhich my invention is especially applicable is fully esterifie'dpolyvinyl cinna'rnate of the above example which is substantially freeof hydroxyl groups. The light-sensitivecinnarnic acid .ester 'coatingsof the invention may contain various sensitizing agents inconcentrations of the order of about 2 to 25 percent, e. g., nitrocompounds of U. S. Patent,2,610,120, triphenyl methane, anthrone,quinone and ketone sensitizers of the Minsk et a1. U. S. patentapplication ,Serial Nos. 207,048 51, filed January 21, 1951, as well asthe thiazole sensitizers of the Robertson' etv al. U. S patentapplication Serial No. 314,806, filed October 15,, 1952. Pigmerits anddyes may also be contained in the coatings for .various purposes which.will be apparent to those skilled in the art. Also, instead of, usingthe pure glycol ethers of the invention to alleviate scumming, I findthat a limited amount of water can be tolerated in the solution used fortreating the developed printing plate, the quantity of which can be bestdetermined by simple experiment under the particular conditions ofoperation.

What I claim is:

1. A process for preparing a printing plate which comprises exposingto aline or half-tone subject a layer of a"cinn'amic acid ester of polyvinylalcoholcoated upon a zinc surface, applying to the surface of theexposed layer an organic solvent in which the cinnarnic acid ester ishighly"soluble"nnt il thecinna'rnic acid "ester has been removed fromthe zinc surface 'only in the unexposed region, and swabbingthe layerwith a member of the class consisting of ethylene glycol m'onomethylether and ethylene glycol monoethyl ether.

.2,i'A.,pro c ss for. preparing. a printing plate whichcomprisesvexposing to a lineior, half-tone'subject a layer qtpolyvinylcinnamate coated upon a zinc surface, applying to. the surface of theexposed-layer an organic solvent in which the polyvinyl cinnamate ishighly soluble untilthe polyvinykcinnamateihas been removed from theZinc. surfaceonly in the unexposed region, aridswabbing the layer ,with.a member of the class consisting of ethy1ene ,gly.c0l1 monomethyl etherand ethylene glycol monoethyl,ether.

. 3., Aprocess for preparing a printing plate which comprisesexppsingtoa line; or half-tone subject a layer of pqly lizl .cinnamate coatedupona zinc surface, applyingtrit;v loroethylene to .thesurface oftheexposed layer until the-polyvinyl .cinnamate has. been removed fromthetZinC Surface only in, the unexposedregion, and swabbingthelayerwith. a. member of the class consisting of ethylene glycolmonomethyl ether and ethylene glycol nio'ndethyl ether.

No references cited.

1. A PROCESS FOR PREPARING A PRINTING PLATE WHICH COMPRISES EXPOSING TOA LINE OR HALF-TONE SUBJECT A LAYER OF A CINNAMIC ACID ESTER OFPOLYVINYL ALCOHOL COATED UPON A ZINC SURFACE, APPLYING TO THE SURFACE OFTHE EXPOSED LAYER AN ORGANIC SOLVENT IN WHICH THE CINNAMIC ACID ESTER ISHIGHLY SOLUBLE UNTIL THE CINNAMIC ACID ESTER HAS BEEN REMOVED FROM THEZINC SURFACE ONLY IN THE UNEXPOSED REGION, AND SWABBLING THE LAYER WITHA MEMBER OF THE CLASS CONSISTING OF ETHYLENE GLYCOL MONOMETHYL ETHER ANDETHYLENE GLYCOL MONOETHYL ETHER.